Field of the Invention
The present invention relates to a circuit configuration for applying a clocked application of a supply voltage to a load.
Such circuit configurations are used, by way of example, in switched-mode power supplies or power factor controllers, in which an inductor is respectively connected to a supply voltage in clocked fashion based upon actuation signals. In such a case, the actuation signals for the first switch, which is connected in series with the load, are produced by the control circuit, whose correct operation requires a supply voltage applied to its voltage supply terminals.
A prior art circuit configuration of such a type is disclosed in European Patent Application 0 585 788 A1, corresponding to U.S. Pat. No. 5,285,369 to Balakrishnan, particularly, in FIG. 3 thereof. In such a case, the circuit configuration is part of a switched-mode power supply, where a series circuit including the primary coil of a transformer and a first switch, in the form of a MOSFET, is connected between output terminals of a bridge rectifier. To actuate the MOSFET, a control circuit is provided. To supply the control circuit with voltage, a depletion-mode MOSFET or a JFET (Junction FET) is connected between the primary coil and the MOSFET, with one supply terminal of the control circuit being connected to a node that is common to the JFET and to the MOSFET in order to output a supply voltage for the control circuit when the MOSFET is off. Furthermore, in addition to the secondary winding of the transformer, the secondary side is provided with an auxiliary winding having a downstream rectifier, the auxiliary winding likewise being used to supply voltage to the control circuit.
When the MOSFET acting as a switch in this apparatus is on, the JFET connected in series with it is also on. When the MOSFET turns off, the JFET also starts to turn off, which results in approximately all of the supply voltage appearing across the JFET. In a transition phase, a current then still flows through the JFET into the control circuit, but this is associated with a high power loss on the JFET.
A secondary-side auxiliary winding having a rectifier, use of which for supplying voltage to a control circuit even without the JFET from the cited print is already in the prior art, requires the use of discrete components whose cost has an adverse effect on the total price of the power supply unit or power factor controller. This is particularly true for switched-mode power supplies in power classes between 5 W and 20 W, which are used to safeguard the xe2x80x9cstandby modexe2x80x9d in electronic appliances and that are subject to an especial cost pressure.
It is accordingly an object of the invention to provide a circuit configuration for applying a supply voltage to a load that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that dispenses with an auxiliary winding and with the use of discrete components for the voltage supply to the control circuit and in which there is no or only very little power loss when providing the supply voltage for the control circuit.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a circuit configuration for clocked application of a supply voltage to a load, the circuit configuration including a first switch having a first control connection, the first switch connected in series with the load, a control circuit connected to the first switch, the control circuit having a first output terminal connected to the first control connection for actuating the first switch and first and second voltage supply terminals, and a second switch having a second control connection to which an actuation signal is supplied, the second switch connected between the load and the first voltage supply terminal.
Accordingly, in addition to the first switch connected in series with the load, the circuit configuration has a second switch that is connected between the load and the first voltage supply terminal of the control circuit. In such a case, when the first switch has turned off, the second switch, which is on and is preferably likewise actuated by the control circuit, takes on the current flowing through the load and supplies it to the control circuit through the first voltage supply terminal thereof. The current flowing through the second switch is preferably buffered in a capacitor connected in parallel with the voltage supply terminals of the control circuit to provide a continuous, only slowly decreasing supply voltage for the control circuit after the second switch has turned off until it next turns on.
In the on state, the voltage drop across the second switch is only small in relation to the total voltage on account of the low conduction resistance, which means that the power loss on the second switch is low. A large part of the voltage drops across the load, with inductive loads involving storage of energy that is subsequently used in a switched-mode power supply or power factor controller.
The second switch can be implemented, almost without additional cost requirement, as an integrated component in the integrated circuit for the control circuit or in the integrated circuit for the first switchxe2x80x94with the control circuit and the first switch also being able to be accommodated in an integrated circuit.
In accordance with another feature of the invention, the control circuit has a second output terminal connected to the second control connection for actuating the second switch.
In accordance with a further feature of the invention, there is provided a capacitor is connected between the first and second voltage supply terminals and in parallel with the control circuit.
In accordance with an added feature of the invention, there is provided a series circuit including the second switch and the control circuit is connected in parallel with the first switch. So long as the first switch in such an embodiment is on, the voltage appearing across the series circuit including the second switch and the control circuit is not sufficient to cause a flow of current, when the second switch is on, into the control circuit and to the capacitor connected in parallel with the control circuit. When the first switch is turned off, the voltage appearing across its load path rises until a voltage has been reached that allows a current to flow to the capacitor through the second switch, which is on. The second switch is preferably likewise actuated using the control circuit, with a second output terminal of the control circuit being connected to a control connection of the second switch.
The second switch can be actuated in various ways. Thus, the second switch can already be on when the first switch is on. Alternatively, the second switch can be turned on only shortly before or at the same time as the first switch turns off in order to supply current to the control circuit for a period of time. The period for which the second switch remains on after the first switch has turned off is dependent, in the control circuit, on the voltage present across the voltage supply terminals or across the capacitor. Preferably, the second switch remains on until the capacitor has been xe2x80x9cfilledxe2x80x9d again with the charge drawn since the last charging operation or until the voltage across the capacitor reaches a prescribable nominal value.
In accordance with an additional feature of the invention, there is provided a diode connected between the first voltage supply terminal and the second switch.
In accordance with yet another feature of the invention, the first switch and the second switch have a common node and a diode is connected between the common node and the first voltage supply terminal of the control circuit.
In accordance with yet a further feature of the invention, there is provided a diode connected between the first voltage supply terminal and the second control connection.
In accordance with yet an added feature of the invention, the second switch is connected in series with the first switch and preferably between the load and the first switch. In such an embodiment, the first voltage supply terminal of the control circuit is connected to the node that is common to the first and to the second switch, preferably, through a diode. When the first and second switches are on, a current flows through the load and through the two switches connected in series therewith. Virtually all of the supply voltage present across the series circuit drops across the load. The voltage dropping across the first switch is not sufficient to cause a flow of current in the control circuit connected in parallel therewith and to the capacitor connected in parallel with the control circuit. If the first switch is turned on and the second switch remains on, the voltage appearing across the first switch rises until, with the second switch on, a current can flow through the load and the second switch to the capacitor to charge it again. The second switch is proportioned such that it has a low conduction resistance in the on state, which means that the power loss across it is low.
In accordance with yet an additional feature of the invention, the supply voltage is supplied at a terminal and a resistor is connected between the terminal and the first voltage supply terminal.
In accordance with again another feature of the invention, there is provided a resistor connected in parallel with the second switch between the load and the first voltage supply terminal.
In accordance with again a further feature of the invention, the first switch has a connection terminal remote from the load and the second voltage supply terminal and the connection terminal are connected to a common terminal at reference ground potential.
The first and second switches are preferably in the form of semiconductor switches, particularly in the form of MOSFETs, with the second semiconductor switch being able to be in the form of a depletion-mode FET or JFET.
In accordance with again an added feature of the invention, the first and second switches are semiconductor switches selected from the group consisting of FETs and IGBTs. The second switch can be a depletion-mode MOSFET or a JFET. The first switch can be an IGBT and the second switch can be a MOSFET.
The invention also covers a method for actuating the first and second switches.
With the objects of the invention in view, there is also provided a switched-mode power supply for applying a supply voltage to a load, the power supply including a first switch having a first control connection, the first switch connected in series with the load, a control circuit connected to the first switch, the control circuit having a first output terminal connected to the first control connection for actuating the first switch and first and second voltage supply terminals, a second switch having a second control connection to which an actuation signal is supplied, the second switch connected between the load and the first voltage supply terminal.
With the objects of the invention in view, there is also provided a method for applying a supply voltage to a load, including the steps of connecting a first switch in series with the load, the first switch having a first control connection, connecting a control circuit to the first switch, the control circuit having a first output terminal and first and second voltage supply terminals, connecting the first output terminal to the first control connection for actuating the first switch, connecting a second switch between the load and the first voltage supply terminal, the second switch having a second control connection, supplying an actuation signal to the second control connection, actuating the first and second switches by turning on the first and second switches at least approximately at the same time and turning off the first and second switches at different times.
In accordance with again an additional mode of the invention, an on duration of the second switch is dependent upon a value of the supply voltage present across the first and second voltage supply terminals.
With the objects of the invention in view, there is also provided a method for applying a supply voltage to a load, including the steps of connecting a first switch in series with the load, the first switch having a first control connection, connecting a control circuit to the first switch, the control circuit having a first output terminal and first and second voltage supply terminals, connecting the first output terminal to the first control connection for actuating the first switch, connecting a second switch between the load and the first voltage supply terminal, the second switch having a second control connection, supplying an actuation signal to the second control connection, actuating the first and second switches by turning on the second switch one of before and at approximately the same time as the first switch is turned off and turning off the second switch after the first switch has been turned off.
The invention also provides for the use of the inventive circuit configuration in a switched-mode power supply or a power factor controller.
With the objects of the invention in view, in a switched-mode power supply applying a supply voltage to a primary coil of a transformer, there is also provided a circuit configuration including a first switch having a first control connection, the first switch connected in series with the primary coil, a control circuit connected to the first switch, the control circuit having a first output terminal connected to the first control connection for actuating the first switch and first and second voltage supply terminals, and a second switch having a second control connection to which an actuation signal is supplied, the second switch connected between the primary coil and the first voltage supply terminal.
With the objects of the invention in view, in a power factor controller applying a supply voltage to an inductor through which an input current flows, there is also provided a circuit configuration including a first switch having a first control connection, the first switch connected in series with the inductor, a control circuit connected to the first switch, the control circuit having a first output terminal connected to the first control connection for actuating the first switch and first and second voltage supply terminals, and a second switch having a second control connection to which an actuation signal is supplied, the second switch connected between the inductor and the first voltage supply terminal.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for applying a supply voltage to a load, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.